The field of plethysmography, the study of the change in size of an organ or limb, often employs inductive sensors as measuring devices. A variety of such sensors have been disclosed in various patents to Goldberg et al., including U.S. Pat. No. 3,560,845. The use of such a sensor to measure cross-sectional area changes in the torso, and thus respiration volume has been discussed in Milledge, J. S., and Stott, F. D., "Inductive Plethysmography--A new Respiratory Transducer", Proceedings of the Physiological Society, January 1997, pages 4-5. By measuring the simultaneous changes in airflow, the changes in the signals from various types of chest and abdominal respiration sensors can be weighted and summed in order to provide an independent measurement of respiration volume. Such calibration procedures were described by Shapiro, A. and Cohen, H. D. (1965) "Transactions of the New York Academy of Science", Vol. 27, page 634. Such techniques were further explored in Konno, K. and Mead, J. (1967) "Journal of Applied Physiology", Vol. 22, page 407. In U.S. Pat. Nos. 4,308,872 and 4,373,534, Watson et al. further describe the use of inductive phethysmography sensors which measure cross-sectional area.
The use of an inductive sensor which circumscribes the torso has been found to have certain inherent disadvantages. Non-invasive respiration inductive sensors are usually only semiquantitative and subject to artifact due to body movement, changes in sleeping position, physical displacement, physical deformation, changes in relative calibration of the chest and abdominal compartments, electrical interference by the chest sensor to the abdominal sensor (and vice-versa), and electrical interference from external electro-magnetic fields including electrical magnetic properties of the torso.
An improved self-inductance sensor is disclosed in U.S. Pat. No. 4,817,625 to Miles (the inventor of the present invention), the disclosure of which is incorporated by reference. This improved sensor includes a band of distensible material and a strap of nondistensible material. The band and strap in combination form a closed loop which circumscribes the object to be measured. A conductor is secured to the band and has two symmetric portions each having a saw-toothed configuration and juxtaposed to one another. The respective portions of the conductor form a plurality of substantially enclosed diamond shaped areas. The change in shape of the areas results in a change in the self-inductance of the conductor. While the sensor of the '625 patent overcomes several of the drawbacks associated with prior sensors, some disadvantages have been found when the upper and lower angle of the enclosed diamond shaped area approaches 90.degree.. When the upper and lower angles are less than 90.degree., inductance decreases when the belt is stretched. When the upper and lower angles are greater than 90.degree., inductance increases when the belt is stretched. However, when the upper and lower angles are stretched or compressed such that they pass through an angle of 90.degree., the signal will reverse and disrupt operation of the sensor. Thus, an advantage exists for maintaining the upper and lower angles either at an angle greater than 90.degree. or less than 90.degree. at all times to ensure efficient operation.
The inductance measurement area of the sensor of the `625 patent does not encompass the entire torso as the conductor does not run through the strap of nondistensible material. Thus, the accuracy of the sensor is dependent on the position of the patient. Thus, an advantage exists for a self-inductance sensor in which the conductor portion entirely circumscribes the object to be measured.
An advantage would also exist for an improved inductive plethysmography respiration sensor which is less sensitive to changes in sleeping position, less sensitive to movement artifact and more sensitive to small changes in chest or abdominal circumference.